1. Field of the Invention
The present invention relates to a laminated capacitor including external electrodes attached at both edge portions of a principal or naked laminated body and a method of such the capacitor for adjusting electrostatic capacity thereof by trimming, and in particular to a trimming method for a laminated ceramic capacitor for use in a resonance circuitry requiring a finely adjusted value of electrostatic capacitance.
2. Description of Related Art
Most laminated capacitors are made of chip-like naked ceramic bodies of a prism and plate shapes, on both edge surfaces of which are attached with terminals for electrical connection, i.e., external electrodes. Among circuit parts of the laminated type, in particular, a laminated capacitor for use in a resonance circuit and a filter, etc., is finely and accurately adjusted in characteristics thereof, by means of, so-called a functional trimming, after it is mounted on a circuit board, especially for adjustment of the frequency characteristic thereof.
FIG. 8 attached shows a laminated ceramic capacitor disclosed in Japanese Patent Laid-Open No. Hei 7-183162 (1995), and it is one of the laminated ceramic capacitors to be adjusted in the capacitance thereof by trimming as mentioned above.
This laminated capacitor has at least a pair of internal electrodes 3 and 3 confronting each other within the naked laminated body 1 of a ceramic laminated body, and those internal electrodes 3 and 3 extend out at the edge surfaces of the naked laminated body 1. The external electrodes 2 and 2 are formed at the edge portions of the naked laminated body 1. The naked laminated body 1, being under the condition that it is separated from the external electrodes 2 and 2, is formed with a trimming electrode 4 on a surface thereof.
With this capacitor, it can be represented by an equivalent circuit of a series connection of capacities which are formed among the pair of internal electrodes 3 and 3 and the trimming electrode 4. This capacitor is designed and produced in such a manner that it has an electrostatic capacitance a little bit larger than a predetermined value, and the capacitance is adjusted to decrease by gradually removing the trimming electrode 4 by irradiation of a laser beam, i.e., by reducing the opposing area defined between the trimming electrode 4 and the internal electrode 3, after it is mounted on the circuit board. Thereby, the capacitance is so adjusted that the resonance frequency is fitted within a predetermined boundary value.
FIG. 9 shows a trimming method of the laminated ceramic capacitor shown in Japanese Patent Laid-Open No. Hei 7-45469 (1995).
Among layers of the naked laminated body 1 formed of a dielectric ceramic material, a plurality of pairs of internal electrodes 6 and 7 are formed so as to oppose each other through the ceramic layers. The internal electrodes 6 and 7 of each pair extend out to the opposing edge surfaces of the naked laminated body 1, respectively, and are connected to the external electrodes 2 and 2 which are formed at respective edge portions of the naked laminated body 1.
In such a laminated capacitor, the opposing area between the internal electrodes 6 and 7 is reduced so as to decrease the capacitance value, by removing a part of the naked laminated body 1 and cutting a portion of the internal electrodes 6 and 7 with the irradiation of the laser beam on the surface thereof. Thereby, the capacitance is so adjusted that the resonance frequency is fitted within a predetermined boundary value.
For improving the efficiency in a production process and in a trimming accuracy with the laminated capacitor mentioned above, at an initial stage of the trimming process in which there is great difference between the actual value in capacitance and the target value thereof, it is desired to reduce the electrostatic capacity abruptly. However, at a final stage of the trimming process in which the actual capacitance value approaches the target value, to reduce the value of electrostatic capacity slowly. Namely, it is preferable to adjust the capacitance roughly at the initial stage of the trimming process and to adjust it finely at the final stage thereof.
However, with the laminated capacitor as mentioned above, the value of the electrostatic capacity decreases almost linearly depending on the decrease in the opposing area of the trimming electrode, since the proportion or rate of reduction in the opposing area of the trimming electrode is almost proportional to that of the value of the electrostatic capacity. Therefore, it is impossible to fulfill both requirements, i.e., the improvement in efficiency of the trimming process as well as in the trimming accuracy.
Further, with the laminated capacitor mentioned above, it is impossible to adjust the electrostatic capacity value by increasing the value thereof since the trimming always functions only to decrease the electrostatic capacity value. Therefore, excessive trimming is not permissible and it cannot be recovered afterward. Furthermore, even when the laminated capacitor produced has an electrostatic capacity value which is only slightly smaller than the designed value due to some reasons, it must be destroyed or disposed of since the electrostatic capacity value of it cannot be brought up to the target value thereof. Therefore, there are drawbacks that the yield rate or factor of the products becomes to be low and brings troubles in that the laminated capacitor must be removed for replacement thereof.